The maximum speed available through an FTTC (fibre to the cabinet) connection is 80Mbps. (That's a nominal figure; in practice the maximum is usually 76Mbps).
Higher speeds require FTTH (fibre to the home) connections, which are only available in under 5% of UK homes.
Someone living alone and, say, streaming TV services over the internet, while browsing the web at the same time, only needs a download speed of around 3Mbps. 5Mbps might be slightly more reliable. 10Mbps is in the 'luxury' class for such a person and anything above that is totally pointless. (That person wouldn't notice any significant difference if he'd got a 100Mbps service. He'd simply have far bigger bills to pay).
A large family, with several teenagers all playing video games at once, while Mum and Dad each separately stream video services, might need around 30Mbps (or even perhaps up to 50Mbps). Anything above that is totally superfluous and will just give them a bigger bill without offering any extra benefit.
So what does your friend actually need 100Mbps for?